This invention is directed to fuel cells, and more particularly, to a comprehensive water management system for use with a fuel cell.
Various fuel cell types exist in the prior art. Each fuel cell type, as defined by its electrolyte, has particular design requirements. In a proton exchange membrane (PEM) fuel cell one requirement is to provide an effective water management system. A PEM fuel cell includes a membrane confined between respective porous cathode and anode electrodes. These electrodes comprise a relatively thin catalyst layer on support plate members where the catalyst layer may be deposited either directly on respective major surfaces of the proton exchange membrane or on a porous support plate, wherein the porous support plate, also known as a substrate is in contact with the major surfaces of the proton exchange membrane. These respective cathode and anode catalyst layers reside at the interfaces between the respective electrode plates and the proton exchange membrane. In general, fuel cells function by supplying a gaseous fuel and an oxidant, through supply means, to the anode electrode and cathode electrode, respectively. These supply means distribute the fuel and oxidant gas as uniformly as possible over the surfaces of catalyzed layers of the respective electrodes. When a PEM fuel cell operates, the electrochemical reaction occurring at the catalyzed anode results in electrons and hydrogen ions being formed at the anode. The electrons flow through an external load circuit and the hydrogen ions flow through the membrane to the catalyzed cathode where they react with oxygen to form water and also release thermal energy.
Typically, fuel cell devices include more than one fuel cell, as described above, arranged in electrical series in a stack. Separator plates separate the individual fuel cells from adjacent cells. Usually, these plates have been nonporous, electrically conductive, impermeable separators. However, this invention uses separator plates that have a fine porous construction enabling water transfer therethrough.
In PEM fuel cell devices, water forms at the cathode catalyst layer. As hydrogen ions travel through the proton exchange membrane, the ions drag water at the anode side and carry the same to the cathode side. This activity causes several problematic events. Water accumulates at the cathode catalyst layer, requiring removal to avoid denying the gaseous oxidant access to the reaction surface thereof. Secondly, the anode side dries out due to the water depletion, thereby requiring measures for water replenishment.
It is known that the porosity of the separator plate along with a system created pressure differential unsatisfactorily achieves water removal from the cathode side and water replenishment to the anode side. U.S. Pat. No. 4,729,932 to McElroy, U.S. Pat. No. 5,503,944 to Meyer and U.S. Pat. No. 4,824,741 to Kunz disclose fuel cell designs that inadequately achieve these principles. Additional problems with PEM fuel cell devices can include excessive water loss at the cathode electrode due to dry inlet oxidant gas as well as evaporation of water into the oxidant stream, particularly at the oxidant inlet. That is, these events without corrective measures can lead to membrane dry out and water insufficiency thereby requiring water replenishment. Some prior art fuel cell stack configurations attempt to circumvent the dry out problem by using a condenser external to the stack. The condenser condenses water from the exiting air stream by heat exchange with a cooling medium such as ambient air and returns the water to the cell stack by way of an external loop. Such an approach adds complexity to the fuel cell power plant system and results in increased power plant weight and volume. Similar arrangements humidifying the oxidant gas entering the cathode electrode area to prevent dry out of the cells are known in U.S. Pat. No. 5,382,478 to Chow et al which uses humidifiers external to the cell stack or in the front segment of the stack not used for electrical reaction. Such humidification techniques using external saturators can only saturate the oxidant gas to an average temperature, but cannot account for the temperature variations that occur within the individual fuel cells of the fuel cell stacks.
Various patents disclose devices addressing these water management problems in fuel cells. For example, U.S. Pat. No. 4,345,008 to Breault discloses an apparatus for reducing electrolyte loss from an electrochemical cell using phosphoric acid as an electrolyte to facilitate the electrochemical reaction. The cell includes a condensation zone at the outlet of the oxidant gas. The condensation zone is an electrochemically inactive portion of the cell because it lacks a catalyst. Accordingly, the condensation zone operates at a cooler temperature than the active-catalyst-containing portion of the cell. To further facilitate a temperature reduction, coolant tube density in the area of the condensation zone is increased. Accordingly, the oxidant gas passes through the condensation zone thereby cooling the oxidant gas and condensing the electrolyte out of the gas. The electrode substrate absorbs the condensed electrolyte and returns it to the active portion of the cell. This fuel cell design does not include a similar humidification region for the prevention of cell dry out. Since the fuel cell does not use a PEM membrane, therefore the requirement for hydrating the inlet reactant gas does not exist. It also requires a more complex manufacturing process for forming the coolant tubes used at the condensation zone.
Further, U.S. Pat. No. 4,769,297 to Reiser et al discloses a water management system for a solid polymer electrolyte fuel cell stack. In U.S. Pat. No. 4,769,297 water feeds into the fuel cell stack in the hydrogen reactant stream where some water evaporates to provide cooling while other water migrates through the stack from cell to cell. Water migrates as a result of being dragged from the anode to the cathode through the electrolyte membrane and via the porous separator plates interposed between two adjacent cells in the stack. The reactant pressure differential pressure maintained between the cathode and anode forces the water through the porous separator plates. The anode support plates provide a large enough surface area from which water is evaporated to perform the cell cooling function. This system uses a condenser for removing water from hydrogen exhaust during fuel cell operation, but does not indicate the supply or withdrawal of water to the oxygen reactant gas.
There exists a need for a water management system for use with a PEM fuel cell stack, which system humidifies the oxidant gas and fuel gas at their inlets and condenses water out of the oxidant gas at its outlet, without the use of exterior humidifiers, for preventing membrane dry out and maintaining water self sufficiency of the fuel cell.
The present invention provides a water management system for the cell anode and cathode and integrates this function with the fuel cell coolant system. This integration allows system operation and control that can handle high current density operation at ambient pressure, and start up and shut down conditions because the system efficiently transfers water throughout the cells, and to and from the coolant system, on an as needed basis and at a rate which may be different for each cell.
The anode and cathode components of the invention consists of a tri-element assembly: 1) a water transport plate (WTP), 2) a bilayer plate, and 3) a catalyst layer/membrane surface. At the anode, water and fuel are required at the catalyst/membrane interface in sufficient quantity to replace water being lost through the membrane by proton drag, or from evaporation into the gas stream, and to supply the fuel necessary for the anode reaction. In the present invention, the WTP provides a full planar surface to the bilayer plate and the WTP acts as a water source that may be augmented by inlet stream water saturation up to about 100% relative humidity. The water supplied to the catalyst/membrane surface may be in the form of either a vapor or liquid. The supply rate will always meet the cell requirement because the water management system capacity, which draws on the coolant system, inherently exceeds total cell demand. This system is also partly self-regulating because the temperature of the WTP and the catalyst/membrane surface of each cell are nearly equal.
At the anode, the means for transporting water from the coolant supply through the WTP to the catalyst/membrane surface while also allowing humidified fuel reactant to reach the catalyst is the bilayer component. Typical bilayer components are described in U.S. Pat. No. 5,561,000.
At the cathode, water removal is the primary concern, but cell inlet oxidant humidification is also important. The water at the cathode catalyst/membrane surfaces comes from two sources: proton drag from the anode and product water from the electrochemical cathode reaction. As current density increases, the amount of water at the catalyst/membrane surface increases proportionally. If this water is not removed, cell performance decreases because the reactant gas will be prevented from reaching the catalyst surface due to flooding of the catalyst layer. The problem becomes more severe when air is used as the reactant instead of pure O2. With the present bilayer configuration, water at the catalyst/membrane surface is not able to form a film or droplets because the fine pore layer of the bilayer is hydrophobic with a mean pore size of about 0.1.0xcexc and 50% porosity. The fine pores force water away from the surface and into the coarse pore layer, which is a less hydrophobic and has a mean pore size of about 20-40xcexc and 65-75% porosity. The coarse layer may even be hydrophilic. Once in the coarse pore layer, water migrates to the WTP as either a vapor or liquid. At the anode, water moves into the WTP, where the entire WTP planar surface is available for water absorption. This minimizes the water transfer distance across the bilayer, which can be critical at high current densities. It also improves the uniformity of reaction within the cell active area.
The WTP operates as a separator plate, an electrical conductor, a reactant humidifier and distributor, a cell cooler and a water transport device. In combination with the bilayer and membrane assembly, the WTP integrates the cell water management and cooling capabilities into each and every cell in the stack so that individual cell variations for cooling and water transport are provided. The bilayer plate acts as a means for facilitating the movement of reactants, water and electrical current through the fuel cell. The bilayer is configured so that the fine pore layer is directly adjacent to the catalyst layer and the coarse pore layer is directly adjacent to the WTP. On the cathode side, the fine pore ensures that water does not form a liquid film that could impede the flow of reactant gas by flooding the catalyst layer. On the anode side, the fine pore provides a means to feed water from the WTP through the coarse pore to the membrane without inhibiting the flow of fuel to the catalyst by transporting the water through the wetproofed layer primarily in the vapor phase, by an evaporative/recondensation process. The coarse pore layer operates in connection with the fine pore layer as a move open structure that can more both liquid water and water vapor while providing sufficient open pore volume to transport reactant gas to the fine pore. In addition to the above operating processes, the tri-element configuration also provides a means for removing water from the gas distribution channels that may result from operational upsets or malfunctions and start-up or shut down conditions.
It is therefore an object of this invention to provide an improved water management system for a PEM fuel cell stack which will provide sufficient water to the reactant gases to humidify the incoming reactant gases while supplying the necessary water required by the anode while removing water from the cathode catalyst and preventing water from blocking any anode or cathode gas passages.
And still another object of this invention is to provide an improved process for the management of water in a PEM fuel cell.